1. Field of the Invention
The invention relates to rechargeable batteries and, in particular, a battery charging system.
2. Description of the Related Art
Batteries are used to power a wide variety of electrical devices. For example, the devices powered by batteries range from a simple flashlight to a complex portable computer. In another example, batteries power devices ranging from a small cellular phone to a relatively large vehicle.
A battery is a power source comprising one or more electrochemical cells, also referred to as voltaic cells. Each voltaic cell comprises two electrodes called a cathode and an anode, with an electrolyte therebetween. Electrochemical reaction between the electrolyte and the electrode causes separation of positive and negative charges, wherein the positive charge accumulate on the anode and the negative charge accumulate on the cathode thereby forming a potential difference. When an external path (circuit) is formed between the electrodes, the potential difference causes the charge to flow (electrical current) in the circuit thereby performing a variety of functions in an electrical device. As the charge is drained from the voltaic cell(s) of the battery, the reaction replenishes the charge up to a point after which the chemical reaction can no longer be sustained. A battery with such “drained” voltaic cell(s) is either replaced or recharged in order to power the electrical device.
Batteries can be grouped into two broad types—a primary type and a secondary type. The primary type comprises disposable batteries, and the secondary type comprises rechargeable batteries. The rechargeable batteries can further be grouped into four types: NiMH (nickel metal hydride), NiCd (nickel cadmium), LiIon (Lithium Ion), and SLA (Sealed Lead Acid) batteries. While the composition of the electrolyte and the electrodes vary among the four types of rechargeable batteries, basic working principle is essentially the same.
In a rechargeable battery, the aforementioned charge-generating electrochemical reaction is reversible in a recharging (or simply referred to as charging) process. The charging process comprises introducing charge from an external source (charger) into the battery such that the charge drives the reverse electrochemical reaction inside the voltaic cell(s). In general, it is desirable to achieve the charging process as quickly as possible. There is a limit, however, on the rate at which the battery can absorb the charge, and such a limit depends on factors such as composition and charge capacity of the battery.
Many conventional battery chargers utilize what is referred to as pulse charging, wherein charge is introduced into the battery in pulses. Pulse charging is known to be an advantageous method of charging in many aspects, including the fact that pulsing of charge followed by a rest period allows the input charge to be absorbed more efficiently. Given such an advantage, traditional pulse chargers further attempts to decrease the charging time by modulating the amplitude of the charge pulses as the charging progresses, based on battery indicators such as voltage. As is known in the art, voltage of the battery being charged is one of the indicators of the state of battery's charge as well as how well the charge is being received by the battery. In particular, a quiescent voltage of the battery (open circuit terminal voltage) is roughly indicative of the charge state of a given battery, and the terminal voltage during a charge pulse relative to the quiescent voltage is indicative of the charge absorption rate. Hence, a traditional pulse charger may modulate the charge pulse amplitude based on the monitored voltages.
Such a pulse charger has drawbacks, including the charger being rather battery specific. The battery specific nature of the conventional pulse chargers arises from the fact that charge pulse amplitude (current amplitude) is modulated based on voltage parameters. As is known in the art, most rechargeable batteries have operating voltages that covers a relatively small range, from several volts to several tens of volts. Thus, a relatively small range of voltage values characterizes a wide variety of batteries whose current rating may differ greatly. Current rating of common rechargeable batteries is known to vary from few milliamps to few amps, a range that covers approximately three orders of magnitude. As an example, a car battery is a 12 volt device that operates with current in the amp range, while a small rechargeable 9 volt battery operates in the milliamp range. Thus attempting to modulate the current amplitude based on these two similar voltage situation is not practical at the least, and may be dangerous in certain situations. Because of such disparity in ranges between voltage and current amplitude, conventional chargers, including pulse chargers, are typically configured to service batteries within a narrower range of current rating.
The conventional pulse charger suffers from an additional drawback even if the charger is configured for a specific group of batteries. The pulse amplitude modulation in response to changes in voltages typically comprises adjusting the amplitude of the pulse. This practice is disadvantageous, for example, in a charging process where the terminal voltage (during pulsing, relative to the quiescent voltage) decreases near the end of the charging process. Such onset of decrease in voltage is indicative of a decrease in charge absorption rate. In response to the voltage decrease, the pulse amplitude is decreased accordingly. In decreasing the pulse amplitude, however, the amount of charge being input per pulse is also decreased. Therefore, a typical conventional pulse charger suffers from reduced charge input as the charge process nears the end.
From the foregoing, there is need for an improved pulse charger that is able to pulse charge a wider range of batteries. There is also a need for an improved charger that is able to maintain a high rate of charge input throughout the charging process.